Current trucks typically use engine driven alternators as the main electrical energy production device and a stack of batteries for energy storage. Known alternators are typically continuously drivenly coupled to the engine crankshaft to provide power when the vehicle engine is running for the vehicle's electrical loads and to charge the battery stack.
Alternators convert mechanical energy into electrical energy and alternators in trucks are understood to have a 100% duty cycle when a truck engine is running because of their continuous coupling to an engine crankshaft. The efficiency of a typical truck alternator can vary between 55% and 75%, depending upon engine speed and electrical load, which leads to a fuel penalty. For example, consider a Class 8 tractor-trailer combination cruising at 60 MPH on the highway. An exemplary engine speed at 60 MPH is approximately 1400 RPM in the top gear. On average, a common vehicle electrical load can consume about 100 amperes at highway speeds. This implies that a 12 volt alternator has to generate 1.2 kW of electrical energy (12 V×100 A=1.2 kW). Taking into account the efficiency of the alternator (e.g., 55% nominal efficiency at an engine speed of 1400 RPM) and belt (e.g., approximately 98% in the case of a belt driven alternator), the total power required from the engine crankshaft to meet this electrical load would be about 2.2 kW (1.2 kW/[0.55×0.98])=2.2 kW. When engine inefficiencies are also considered, an efficiency calculation can be as follows:(engine efficiency)×(belt efficiency)×(alternator efficiency)=(40%)×(98%)×(55%)=21.6% efficient
Class 8 Trucks typically have a bank of 4 batteries for starting. Trucks that have an electric powered parked heating ventilation and air conditioning (HVAC) system often use an additional battery pack consisting of a set of 4 deep cycle batteries. Typical deep cycle batteries in land vehicles are sized at 105 Amp-hrs, last about 500 cycles, and cannot be fully discharged without damaging the battery. All the electrical loads on the truck are met by the battery packs which are kept charged by the alternator. The electrical loads include all of the so-called hotel loads, such as coffee maker, microwave, stereo, lamps, relays, parked HVAC loads, etc.
Some auxiliary power systems, or units (APUs) are powered by a small diesel internal combustion engine. Although they can meet the electrical requirements of a truck or other vehicle, such as when the vehicle is parked, diesel engine APUs require consistent maintenance and have significant emissions, which can require particulate filters to meet emission standards. While quieter than idling the main engine, diesel engine APUs are still relatively noisy and create vibrations that potentially can keep drivers from sleeping soundly. Although they can be used as an APU, these factors make them less desirable for use as a vehicle auxiliary power unit.
A need exists for improvements in vehicle power systems and auxiliary power systems to power vehicle hotel electrical loads.